Bottom Line:
The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer.When the sensors are exposed to relative humidity ranging from 10-80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks.Our sensors show excellent repeatability and stability, over a period of several weeks.

ABSTRACTWe report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10-80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.

f1: (a) Cross sectional view (not to scale) and (b) Optical micrograph of the CMOS device (the scale bar at top right is 200 μm).

Mentions:
A key component of our resistive humidity sensor is the CMOS MEMS based microhotplate (μHP) structure. Details of the μHP structure are reported elsewhere3334. A cross section view of the sensor structure is shown in Fig. 1(a). An optical microscope image of the fabricated device is shown in Fig. 1(b). The silicon die measures 1 mm × 1 mm. It is designed using a 1.0 μm Silicon on Insulator (SOI) CMOS process technology and fabricated in a commercial foundry, followed by deep reactive ion etching (DRIE) to release the thin membrane. The processing employs SOI wafers with 1 μm buried oxide, 0.25 μm SOI layer and 3 metallization layers. The μHP structure typically consists of an embedded, 0.3 μm thick, resistive tungsten microheater (metal layer 1), 0.3 μm thick heat spreader plate (metal layer 2), and a top gold layer for interdigitated sensing electrodes (IDEs). The diameters of the circular heater and membrane structures are 250 and 600 μm, respectively. The heater is fabricated during the CMOS process while the top gold electrodes and corresponding tracks are deposited as a post CMOS process in the same commercial foundry. The tungsten heater controls the operating temperature of the membrane and graphene-based sensing layer. The IDEs underneath the sensing layer are used to measure the change in resistance due to humidity exposure. Use of tungsten in the heater allows the device to operate at a very high temperature (up to 750 °C), if required, for example, when oxide-based sensing materials are used. Gold is used as the electrode material because of its chemical inertness (and hence, unchanged conductivity over prolonged use under various temperature and humidity conditions) compared to commonly used aluminum in this SOI process. The silicon underneath the μHP is etched away, using the dioxide layer as the etch stopper, at a wafer level by DRIE technique. This forms a 4.5 μm silicon dioxide: SiO2 (4 μm)/silicon nitride: Si3N4 (0.5 μm) membrane structure onto which the microheater and electrodes are suspended. The membrane structure reduces DC power consumption of the sensing device to <5 mW when used for humidity sensing in this work. The heating temperature is uniformly confined over the microheater region, due to the buried heat spreader33. The temperature decreases rapidly away from the heater region and is at close to room temperature at the membrane rim, allowing reliable temperature independent on-chip circuit performance35.

f1: (a) Cross sectional view (not to scale) and (b) Optical micrograph of the CMOS device (the scale bar at top right is 200 μm).

Mentions:
A key component of our resistive humidity sensor is the CMOS MEMS based microhotplate (μHP) structure. Details of the μHP structure are reported elsewhere3334. A cross section view of the sensor structure is shown in Fig. 1(a). An optical microscope image of the fabricated device is shown in Fig. 1(b). The silicon die measures 1 mm × 1 mm. It is designed using a 1.0 μm Silicon on Insulator (SOI) CMOS process technology and fabricated in a commercial foundry, followed by deep reactive ion etching (DRIE) to release the thin membrane. The processing employs SOI wafers with 1 μm buried oxide, 0.25 μm SOI layer and 3 metallization layers. The μHP structure typically consists of an embedded, 0.3 μm thick, resistive tungsten microheater (metal layer 1), 0.3 μm thick heat spreader plate (metal layer 2), and a top gold layer for interdigitated sensing electrodes (IDEs). The diameters of the circular heater and membrane structures are 250 and 600 μm, respectively. The heater is fabricated during the CMOS process while the top gold electrodes and corresponding tracks are deposited as a post CMOS process in the same commercial foundry. The tungsten heater controls the operating temperature of the membrane and graphene-based sensing layer. The IDEs underneath the sensing layer are used to measure the change in resistance due to humidity exposure. Use of tungsten in the heater allows the device to operate at a very high temperature (up to 750 °C), if required, for example, when oxide-based sensing materials are used. Gold is used as the electrode material because of its chemical inertness (and hence, unchanged conductivity over prolonged use under various temperature and humidity conditions) compared to commonly used aluminum in this SOI process. The silicon underneath the μHP is etched away, using the dioxide layer as the etch stopper, at a wafer level by DRIE technique. This forms a 4.5 μm silicon dioxide: SiO2 (4 μm)/silicon nitride: Si3N4 (0.5 μm) membrane structure onto which the microheater and electrodes are suspended. The membrane structure reduces DC power consumption of the sensing device to <5 mW when used for humidity sensing in this work. The heating temperature is uniformly confined over the microheater region, due to the buried heat spreader33. The temperature decreases rapidly away from the heater region and is at close to room temperature at the membrane rim, allowing reliable temperature independent on-chip circuit performance35.

Bottom Line:
The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer.When the sensors are exposed to relative humidity ranging from 10-80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks.Our sensors show excellent repeatability and stability, over a period of several weeks.

ABSTRACTWe report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10-80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.